chemistry and energy

Light Energy Review How is light energy produced?.  Electrons release light energy when they fall from a high energy level to a lower energy..  We’re now going to talk about energy r

How are they related?

Energy Encountered Daily

Light Energy Review

 How is light energy produced?

 Electrons release light energy when they fall from

a high energy level to a lower energy.

 We’re now going to talk about energy

released or used in a chemical reaction Heat energy

Thermochemistry

 The study of heat used or released in a

chemical reaction

 Let’s investigate heat as it compares to

temperature using the Heat vs Temperature Handout

Specific Heat Calculations

Specific Heat

 Specific heat of water = 1 cal /g o C or

= 4.184 J / goC

 Specific heat of most metals = < 1 J / goC

 Do metals heat slowly or quickly compared to water?

 Do metals stay warm longer or shorter than water?

q = (120.0g)(4.184 J/goC)(22.0oC) =

Practice Problem

 How much heat (in kJ) is given off when 85.0

g of lead cools from 200.0oC to 10.0 oC?

(Specific heat of lead = 0.129 J/g oC)

-How Do Chemical Reactions Create Heat energy?

 Consider the combustion of gasoline (octane)

2 C8H18 +25 O2  16 CO2 +18 H2O

 Potential Energy: Stored energy

 Potential energy is stored in the bonds of the

reactant s and the products

 When bonds are broken, the energy is available

 When produce bonds form, some energy is used in these bonds

 The excess energy is released as heat

Kinetic Energy

 Directly related to temperature

Is Heat Used or Released?

 Endothermic reactions used heat from the surroundings

Endothermic Reactions

 Decrease in kinetic energy  decrease in temperature  heat will transfer from the environment to the system resulting in a cooler environment

 Absorbs heat from its surrounding

 The system gains heat

 Positive value for q

 ∆H = q = >0

 Hproducts > Hreactants

Exothermic Reactions

 Increase in kinetic energy  increase in

temperature of system heat released to the environment resulting in a hotter

environment

 Releases heat to its surroundings

 The system loses heat

 Negative value for q

 ∆H = q = <0

 Hproducts < Hreactants

 Heat content for systems at constant

pressure

 Symbol is H

 Terms heat and enthalpy are used

interchangeably for this course

 ∆H = q = m C ∆T

 Heat moves from to _

Thermochemical Equations

 An equation that includes the heat change

 Example: write the thermochemical equation for this reaction

 CaO(s) + H2O(l) → Ca(OH)2(s) ∆ H = -65.2 kJ

CaO(s) + H2O(l) →Ca(OH)2(s) + 65.2 kJ

Stoichiometry and Thermochemistry

Tin metal can be extracted from its oxide

according to the following reaction:

SnO2(s) + 4NO2(g) + 2H2O(l) + 192 kJ  Sn(s) + 4HNO3(aq)

How much energy will be required to

extract 59.5 grams of tin?

How to solve

1. Use your stoichiometry

2. Treat heat as a reactant or product

SnO2(s) + 4NO2(g) + 2H2O(l) + 192 kJ  Sn(s)

+ 4HNO3(aq)

59.5 g Sn 1 mol Sn 192 kJ

1 g Sn 1 mol Sn

If an Object feels hot, it means it is

undergoing a change with a ∆ H that is:

If the object feels hot, it means it is undergoing:

How does ice melt?

Molar Heat of Fusion

Molar Heat of Vaporization

 ∆H vap

Molar Heat of Condensation

condenses

 ∆H cond

 ∆H vap = -∆H cond

Phase Change Diagram for Water

Phase Change Diagram

The House that Heats Itself

 http://www.sciencefriday.com/videos/watch/10007

 Method used to determine the heat involved

in a physical or chemical change

 Relies on the law of conservation of energy

Calorimeter

Simple Calorimeter

Calorimetry Math

 Heat lost by the system = -q

mC∆T = q

∆T = T f –T i , m = mass, C = specific heat

q gained by water = q lost by system

 q water = - q system

 mC∆T = -mC∆T

(mass H2O)(spec heat H2O)(∆T H2O) = - (mass sys)(spec heat sys)(∆T sys)

Standard Heat of Reaction

 Heat change for the equation as it is written

compound is formed from its elements in

Hess’s Law

that may be too slow or too fast to collect data from.

result in the desired reaction Add the ΔH for these reactions in the same way.

 ∑∆H total = ∑∆H products - ∑∆H reactants